The pending PR #1351 needs to test some of the console commands to cover all methods in \App\Images as certain methods are only invoked by those commands. (For example, only the command lychee:generate_thumbs checks for the existence of size variants.)

When I tried to write test cases for the console commands, I noticed that

• the existing tests were never executed, because the name of test class did not end with ...Test.
• a composer dependency to Mockery was missing which is required to mock and capture the console output of the commands
• two commands were so fundamentally broken that they could not even be started (oh, oh)

This PR repairs all that in order to keep it out of PR #1351. Also note that I have already split the existing tests for console command into three files, because the tests for the command lychee:generate_thumbs and lychee:takedate will significantly be increased by PR #1351 and hence the deserve their own files already.